All Tables

Table 1: The sample of the 17 young BDs surveyed in the XEST. Column (2) gives the reference of the discovery paper. Col. (3) give the 2MASS counterparts. Column (7) gives the references for the spectral type (Col. 4), the optical extinction (when not available derived from $A_{\rm J}$ using Rieke & Lebofsky 1985, Col. 5), and the reference of the equivalent width of H $\alpha$ ( $EW({\rm H}\alpha )$ , given in Col. 6) if it is different. Negligible optical extinction in Col. (5) are indicated by 0. The effective temperature in Col. (8) has been computed from the spectral type using the temperature scale $T_{\rm eff}=3841.94-141.17 \times {\rm SpTyp}$ , which is valid for young stars with M spectral type (Guieu et al. 2006). The visual extinction in Col. (5) is taken when available from the literature; for 2MASS J0421 and 2MASS J0422, we averaged the visual extinction computed from J-H, $H-K_{\rm S}$ , and the spectral type, using a dwarf sequence (compiled from the literature; e.g., Leggett et al. 1998). The bolometric luminosity of the substellar photosphere in Col. (9) has been computed from I, J-band magnitudes and $A_{\rm V}$ (see Guieu et al. 2006); for 2MASS J0414 and MHO 4 (without I-band magnitude available) the reference for the luminosity is Luhman (2004) and Briceño et al. (2002), respectively. Column (10) indicates accreting sources based mainly on $EW({\rm H}\alpha )$ (see Sect. 6.2). The last column indicates detection in X-rays (this work; see Table 2). References: B98 = Briceño et al. (1998); B02 = Briceño et al. (2002); G06 = Guieu et al. (2006); L04 = Luhman (2004); L06 = Luhman (2006); M01 = Martín et al. (2001); M05 = Muzerolle et al. (2005a).
Table 2: Detection list of young BDs in the XEST. Column (1) numbers correspond to labels in Fig. 1. Columns (2)-(4) give the BD, satellite and target names, respectively. The naming of XMM-Newton sources in Col. (5) follows the convention of Güdel et al. (2007), where the two and three digits code for the field and the source number in this field, respectively. Columns (6)-(9) give X-ray source positions, total positional uncertainties, and distance to the 2MASS position, respectively. References: [FGMSD03] = Favata et al. (2003), [BFR03] = Bally et al. (2003). There is only pn data for X-ray source #1 as it falls in the gap of the MOS CCDs in window mode in this observation. X-ray source #4b is affected by pn gap. For X-ray source #9 only MOS data are available.
Table 3: Spectral properties of BDs obtained from spectral fitting. To fit the spectra (Fig. 4), we used a WABS absorption model (Morrison & McCammon 1983) combined with one or two MEKAL optically thin thermal plasma model (Mewe et al. 1995) with 0.3 times the solar elemental abundances. MEKAL plasma models were computed rather than interpolated from a pre-calculated table. We used $\chi ^2$ statistics with standard weighting. Column (4) gives the net source counts collected by the instrument given in Col. (3). Confidence ranges at the 68% level ( $\Delta \chi ^2=1$ ; corresponding to $\sigma =1$ for Gaussian statistics) are given in parentheses. The value of reduced $\chi ^2$ and $\nu$ , the degrees of freedom, are indicated in Col. (10). The emission measures in Cols. (8) and (9) and the X-ray luminosity in the 0.5-8 keV energy range corrected for absorption in Col. (11) were computed assuming a distance of 140 pc for the TMC. The X-ray fractional luminosity, $\eta=\log (L_{\rm X}/L_*)$ , is given in the last column. For CFHT-BD-Tau 1, the second line gives an estimate of the quiescent X-ray luminosity derived for the light curve fit (see Sect. 3).
Table 4: Spectral properties of BDs obtained from quantile analysis. Column (3) gives the instrument name (MOS stands for MOS1+MOS2). Column (4) and (5) indicate the exposure and the net source counts collected by this instrument (i.e. for MOS the exposure average and the sum of net source counts). Column (6) gives the energies below which the net counts are 25%, 50%, and 75% of the source net counts in the 0.5-7.3 keV energy range. The resulting position in the quantile diagram (Fig. 5) is given in Col. (7). The observed optical extinctions (see Table 1) were used to disentangle temperature double solutions in the quantile diagram (#2 on pn), and to estimate the hydrogen column density (Col. 8) for sources with low constraint in the quantile diagram (#2, #6 and #7 on pn, and #9 on MOS), using the relation $N_{\rm H} = 1.6 \times 10^{21} A_{\rm V}$ cm^-2 mag^-1 (Cardelli et al. 1989; Vuong et al. 2003); these values are between brackets in Col. 8. Where negligible optical extinction was measured, we adopted $N_{\rm H} = 0.1 \times 10^{21}$ cm^-2. When no constraint on the plasma temperature was obtained, we adopted 1 keV (value between brackets in Col. 9). The X-ray luminosity in the 0.5-8 keV energy range corrected for absorption in Col. (10) was computed assuming a distance of 140 pc for the TMC. The X-ray fractional luminosity, $\eta=\log (L_{\rm X}/L_*)$ , is given in the last column.
Table 5: Upper limits to the BD X-ray luminosities. The hydrogen column density in unit of 10²¹ cm^-2 given in Col. (3) is obtained from the optical extinction using the relation $N_{\rm H,21} = 1.6 \times A_{\rm V}$ cm^-2 mag^-1 (Cardelli et al. 1989; Vuong et al. 2003). Exposure times in Col. (4) are for summed EPIC (pn+M1+M2) data, in units equivalent for a pn-on-axis observation. Columns (5) and (6) give the upper limits at the 95% confidence level for net counts in the 0.5-2 keV energy range and the X-ray luminosity in the 0.5-8 keV energy range corrected for absorption, respectively. The upper limit to the X-ray fractional luminosity, $\eta=\log (L_{\rm X}/L_*)$ , is given in the last column.
Table A.1: Archival Chandra observations which surveyed serendipitously the TMC BDs. Column (1) gives the name of the instrument used: "I'' ( $17\hbox {$^\prime $ }\times 17\hbox {$^\prime $ }$ field of view) and "S'' ( $8.5\hbox {$^\prime $ }\times 25.5\hbox {$^\prime $ }$ field of view) for imaging and spectroscopy ACIS CCD (Garmire et al. 2003), respectively. Column (8) give the name of the TMC BD whitin the Chandra field of view. Column (9) indicates whether we use this archival data to supplement the XEST or not (see following notes). Notes: in the ACIS-S observation #3364, the TMC BD KPNO-Tau 2 was located 13 $^\prime$ off-axis on ACIS-S2, and it is not detected; we obtained here a better constraint on the X-ray luminosity of this BD using the sum of the two XMM-Newton exposures, rather than this ACIS-S observation. In the ACIS-S observation #4488, the TMC BD 2MASS J0421 was located 5.7 $^\prime$ off-axis on ACIS-S3, the pipeline detection algorithm found no source at this location.